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. 2024 Mar 7;15(9):2606-2615.
doi: 10.1021/acs.jpclett.4c00044. Epub 2024 Feb 29.

Deciphering Density Fluctuations in the Hydration Water of Brownian Nanoparticles via Upconversion Thermometry

Fernando E Maturi  1   2 , Ramon S Raposo Filho  1 , Carlos D S Brites  1 , Jingyue Fan  3 , Ruihua He  3 , Bilin Zhuang  4 , Xiaogang Liu  3 , Luís D Carlos  1
Affiliations

Deciphering Density Fluctuations in the Hydration Water of Brownian Nanoparticles via Upconversion Thermometry

Fernando E Maturi et al. J Phys Chem Lett. .

Abstract

We investigate the intricate relationship among temperature, pH, and Brownian velocity in a range of differently sized upconversion nanoparticles (UCNPs) dispersed in water. These UCNPs, acting as nanorulers, offer insights into assessing the relative proportion of high-density and low-density liquid in the surrounding hydration water. The study reveals a size-dependent reduction in the onset temperature of liquid-water fluctuations, indicating an augmented presence of high-density liquid domains at the nanoparticle surfaces. The observed upper-temperature threshold is consistent with a hypothetical phase diagram of water, validating the two-state model. Moreover, an increase in pH disrupts the organization of water molecules, similar to external pressure effects, allowing simulation of the effects of temperature and pressure on hydrogen bonding networks. The findings underscore the significance of the surface of suspended nanoparticles for understanding high- to low-density liquid fluctuations and water behavior at charged interfaces.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Infographic of the various techniques used to investigate anomalies in liquid water across different length scales. The temperature dependence of the H-bond networks has been explored at different length scales. While X-ray and neutron scattering, numerical simulations, and Kerr effect, dielectric, terahertz, ultraviolet–visible, nuclear magnetic resonance, and Raman spectroscopies operate at the length scale of hydrogen atoms and water molecules, SH imaging works at longer scales. Light scattering and luminescence nanothermometry, as shown in this work, can also be used up to a submicrometer length scale.
Figure 2
Figure 2
Solvent effect and size dependence in the Brownian velocity of UCNPs. (a) Temperature-dependent Brownian velocity of the 15 nm UCNPs suspended in EtOH, H2O, and D2O. The gray arrow highlights the existence of a crossover temperature in the water-suspended UCNPs around 330 K, indicating the anomalous behavior of water. (b) Temperature-dependent Brownian velocity of different-sized UCNPs (15–106 nm) at pH 5.10. (c) Size-dependent crossover temperature of the nanofluids from panel b, where the red dashed line is a guide for the eyes, highlighting the operating range of sizes that can be used to probe the different motifs of liquid water. The inset presents the dependence of Tc on the surface/volume ratio (S/V = 6/d). The lines are guides for the eyes.
Figure 3
Figure 3
Hypothetical phase diagram of liquid water. (a) Coexistence of HDL (red) and LDL (blue) domains near the Widom line (W). Below W, LDL dominates with fluctuations in HDL domains, whereas above W, HDL dominates with LDL fluctuations. The white star represents the liquid–liquid critical point. With greater distance from the critical point, fluctuations decrease in size, as indicated by the blobs. The gray line outlines the “funnel of life”, where water exhibits unusual properties crucial for maintaining life. Outside the funnel, at higher temperatures, only local fluctuations occur in the HDL liquid (indicated by small blue dots on the red background). Reproduced with permission from ref (8). Copyright 2019 Springer Nature. (b) Close-up of the shaded area in panel a showing the upper-temperature limit of the “funnel of life” at ambient pressure, corresponding to crossover temperature Tc (diamond), and illustrative schemes of the temperature dependence of high- to low-density liquid fluctuations.
Figure 4
Figure 4
Correlation among the crossover temperature, pH, and ζ potential. (a) Effect of pH on the Brownian velocity of UCNPs with diameters within the operating range of the nanorulers, as defined in Figure 2c. The dashed lines are the best linear fits at each pH for T < Tc and the same linear fit for all of the pH values for T > Tc (r2 > 0.98 for all samples). Tc as a function of (b) pH and (c) |ζ|. The solid lines are guides to the eyes.
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